
# 70. 爬楼梯



def main(n):
    dp = [0]*(n+2)
    dp[0] = 0
    dp[1] = 1
    dp[2] = 2
    for i in range(3,n+1):
        dp[i] = dp[i-1] + dp[i-2]
    return dp[n]

if __name__ == "__main__":
    n = int(input())
    result = main(n)
    print(result)